This invention relates generally to trailer brake systems. More specifically, this invention relates to a change-over valve for a two-line trailer brake system with at least one brake circuit. Such a change-over valve includes a housing and a slide sealingly guided therein, the slide being moved into a normal position wherein a supply tube leading to a compressed air supply tank for the brake circuit is closed and being detained in the normal position by a piston loaded with compressed air from another supply tube. The slide is manually slideable into a maneuvering position wherein the brakes of the two-line trailer brake system are released.
Such a change-over valve is shown and described in published German patent application Ser. No. DT-OS 2 135 924, which also discloses a two-line, two-circuit trailer brake system with two separate brake valves. One of these two brake valves is operatively connected via this change-over valve with spring-loaded brake cylinders provided in one brake circuit only. Upon the connection and flow of compressed air through the supply tube of the trailer brake system, a slide of the change-over valve is moved into the normal position and detained there by a piston arranged on the slide, unless, of course, the slide was already in the normal position. On decoupling of the trailer, the spring loaded brakes are actuated in the usual way. For maneuvering purposes the slide of the change-over valve can be moved manually into the maneuvering position wherein the supply tube of the trailer brake system is disconnected and a cylinder of the spring-loaded brakes is charged with compressed air from the supply tank of one of the brake circuits.
In published German patent application Ser. No. DT-OS 2 152 996 two brake valves for a two-circuit trailer brake system are shown and described. These brake valves are only mounted together and otherwise have two emergency brake pistons that cause lock braking of the trailer on its decoupling in the usual way. In published German patent application Ser. No. DT-OS 2 148 572 there is shown a single trailer brake valve formed as a two-circuit device. One of the two brake circuits has an emergency brake piston, which acts only on one brake circuit. In published German Patent Application DT-OS 2 158 229 there is shown a similarly constructed two-circuit trailer brake valve with a single emergency brake piston which, however, acts on both brake circuits.